Patent Document 1 discloses a failure diagnosis method for a power supply relay, i.e., a semiconductor relay for shutting off power supply, included in a motor controller. In this failure diagnosis method, the power supply stabilizing capacitor in the inverter circuit is charged and the turning on and off of the power supply relay is controlled so as to detect a short circuit failure or a disconnection failure therein. Patent Document 2 discloses a pre-charge circuit for a power supply stabilizing capacitor included in a motor driving device, and the pre-charge circuit charges the power supply stabilizing capacitor via a resistor.